Introduction

Melanocytes are derived from neural crest cells. Precursor cells migrate from the neural crest to the epidermis in utero. The melanocytes develop dendritic cell processes and produce intracytoplasmic pigment granules. They send their dendritic processes up in between the keratinocytes and transfer the melanin to the keratinocytes.

Normal melanocytes inhabit the basal layer of the skin and the bulb of the hair follicle.

The two terms below are used to classify some of the different types of melanomas.

Junctional activity: the proliferation of nests or th èques (aggregations) of melanocytes along the dermo-epidermal junction.

Compound tumor: indicates that there is a junctional and an intradermal component to the tumor.

Melanocytic neoplasms can either be benign or malignant. To distinguish benign from malignant in dogs the following should be evaluated:

the anatomic location of the tumor

the cytologic features

mitotic activity

cell morphology

All histopathologic sections should be bleached prior to evaluation as the nuclear morphology may be obscured by the intracytoplasmic melanin granules.

Melanocytoma

Introduction

Definition: A benign tumor arising from melanocytes either in the epidermis, dermis, or adnexa, primarily the external root sheath of the hair follicles.

Synonyms: Cutaneous Melanoma, Dermal Melanoma, Benign Melanoma

Epidemiology

Sex

N

Percent

Female

637

17% (21%)

Female Spayed

1254

32% (33%)

Male

1084

28% (25%)

Male Castrated

881

23% (21%)

( Normal Population %)

Breeds at
Increased Risk

N

Probability

OR

95%
Confidence
Interval

Vizsla

70

<0.0001

7.5

5.83

9.61

Irish Terrier

7

0.0002

6.7

3.06

14.52

Miniature Schnauzer

306

<0.0001

6.4

5.67

7.23

Standard Schnauzer

137

<0.0001

5.0

4.21

6.00

Bedlington Terrier

6

0.0021

4.9

2.12

11.13

Australian Terrier

8

0.0016

3.9

1.89

7.89

Silky Terrier

11

0.0002

3.9

2.09

7.08

Chesapeake Bay Retriever

34

<0.0001

3.6

2.54

5.08

Airedale Terrier

65

<0.0001

3.6

2.76

4.57

Shar-Pei

75

<0.0001

3.4

2.67

4.27

Doberman Pinscher

323

<0.0001

3.3

2.96

3.73

Irish Setter

87

<0.0001

3.3

2.66

4.11

Rhodesian Ridgeback

31

<0.0001

3.2

2.25

4.64

Bloodhound

11

0.0018

3.0

1.63

5.46

Giant Schnauzer

13

0.0013

2.8

1.59

4.83

Norfolk/Norwich Terrier

5

0.0482

2.6

1.07

6.38

Brittany

49

<0.0001

2.6

1.92

3.42

Golden Retriever

538

<0.0001

2.1

1.94

2.34

Bouvier des Flandres

15

0.0139

2.0

1.22

3.42

Rottweiler

156

<0.0001

1.9

1.59

2.20

Cairn Terrier

32

0.0027

1.8

1.26

2.56

Chow Chow

26

0.0090

1.7

1.17

2.57

Scottish Terrier

41

0.0158

1.5

1.10

2.04

Breeds at
Decreased Risk

N

Probability

OR

95%
Confidence
Interval

German Shepherd

113

0.0001

0.7

0.58

0.84

Mixed Breed

673

<0.0001

0.7

0.62

0.73

Labrador Retriever

184

<0.0001

0.6

0.49

0.66

Lhasa Apso

24

0.0029

0.6

0.38

0.84

English Setter

7

0.0355

0.5

0.22

0.98

German Shorthaired Pointer

11

0.0039

0.4

0.25

0.81

Great Dane

12

0.0017

0.4

0.25

0.78

West Highland White Terrier

16

0.0002

0.4

0.26

0.70

Miniature Poodle

42

<0.0001

0.4

0.31

0.58

Cocker Spaniel

74

<0.0001

0.4

0.33

0.53

Collie

13

0.0002

0.4

0.23

0.69

Greyhound

6

0.0102

0.4

0.17

0.84

English Springer Spaniel

23

<0.0001

0.4

0.25

0.56

Pug

6

0.0056

0.4

0.16

0.81

Beagle

29

<0.0001

0.4

0.25

0.51

Toy Poodle

5

0.0041

0.3

0.14

0.78

Weimeraner

5

0.0004

0.3

0.11

0.64

Border Collie

3

0.0093

0.3

0.08

0.82

Shih Tzu

15

<0.0001

0.3

0.15

0.43

Cardigan Welsh Corgi

2

0.0213

0.2

0.06

0.94

Basset Hound

9

<0.0001

0.2

0.12

0.44

Pomeranian

3

0.0007

0.2

0.07

0.65

Shetland Sheepdog

15

<0.0001

0.2

0.12

0.33

Keeshond

3

0.0004

0.2

0.06

0.61

Boston Terrier

3

<0.0001

0.1

0.05

0.44

Norwegian Elkhound

1

0.0062

0.1

0.02

0.88

Maltese

2

<0.0001

0.1

0.03

0.45

Jack Russell Terrier

2

<0.0001

0.1

0.03

0.42

Husky

5

<0.0001

0.1

0.04

0.21

Bichon Frise

2

<0.0001

0.1

0.02

0.28

Dalmatian

2

<0.0001

0.1

0.02

0.26

Old English Sheepdog

1

<0.0001

0.1

0.01

0.37

Site

Percent

Head

28.5%

Forelimb

20.5%

Hindlimb

12.4%

Thorax

9.8%

Abdomen

7.9%

Back

6.8%

Multiple

5.3%

Neck

4.1%

Perineum

2.2%

Scrotum

1.3%

Tail

1.2%

Clinical Presentation/Physical Exam Findings

Size varies for these tumors; they can be smaller than 0.5 cm up to several centimeters in diameter.

These tumors are often solitary, dome-shaped, and pigmented.

They appear smooth and uniform.

Ulceration of the overlying epidermis is uncommon but may be seen occasionally.

The lesions have only a small amount of hair on the surface of the skin.

Some, regardless of the name, can lack melanin and are pale to dark red.

Tumor Pathology

Gross Findings

The mass of the tumor resides in the dermis, but often extends into the subcutaneous tissue.

Edges are well-demarcated from the surrounding dermis and adipose tissue

This is especially evident in highly pigmented tumors.

Colors can vary from jet black to slate gray; the melanin-free tumors can appear pale to dark red.

Some portions of the tumor may contain pigmented and non-pigmented areas interspersed with each other (variegated).

Microscopic Findings

Histopathology

In the lower part of the epidermis there may be nests of neoplastic cells but

Some neoplasms are completely confined to the dermis.

The amount of melanin within the cytoplasm of the neoplastic cells is variable.

Some may be completely melanin-free.

The cell morphology consists of one or more of the following cell types:

small spindle cells

large spindle cells

epithelioid

polygonal

round

There is littlecytologic or nuclear atypia and few mitotic figures (<3 per 10 HP fields).

In the dermis there may be areas of neuroidal differentiation

Balloon Cell Melanocytoma: This uncommon variant is composed of large, round and epithelioid cells that have abundant pale, finely granular-to-foamy cytoplasm. Melanin granules can be very difficult to find in these cells.

Clinical Behavior

The site of origin and histopathologic appearance is what distinguishes melanocytomas from the more serious malignant melanoma. Wide surgical excision is curative for the cutaneous form.

Melanoacanthoma

Introduction

Definition: A tumor with features of a compound melanocytoma and a benign epithelial neoplasm.